A method for quality control is known from U.S. Pat. No. 6,872,895. The known method determines the conditions at the base of a stretch-blow-molded plastic container as indication for a successfully and correctly executed stretch-blow-molding process. However, this known method only determines the mass or volume of the material in the base of the plastic container. To this end, the absorption characteristics of the whole base or of a selected area of said base are determined and, on the basis of this determination, conclusions are drawn with regard to the mass or the volume of the material which still exists in the base after the stretch-blow-molding process. The features taken into account in the absorption measurement comprise, however, also features, such as minor cloudiness or the like, which do not have any influence on the quality of the plastic container and which may corrupt the measurement. Meaningful absorption measurements are, moreover, comparatively complicated. An area-integrating absorption measurement with a non-spatially-resolving sensor is not very meaningful because of the non-linear characteristics of the law of absorption, and, when a spatially-resolving sensor is used, such measurements are extremely complicated, especially in the infrared light region.
Furthermore, it is known e.g. from DE 199 14 028 to determine the quality of a stretch-blow-molded plastic container in that the outer contours of various areas of the container and the ratio of said contours to one another are determined in a transmitted light process. This method does, however, not allow a reliable detection of all deviations.
Furthermore, a great variety of wall thickness determination methods of stretch-blow-molded plastic containers is known, one of said methods being known e.g. from DE-A-101 16 665. A wall thickness measurement must, however, be executed at many different points of the plastic container so as to be meaningful for the quality of the container in its entirety. The method described in this publication can be used for determining, analogously to the wall thickness, also other parameters, such as the density. The known method is therefore comparatively complex.